PROJECT SUMMARY/ABSTRACT Cancers are a leading cause of mortality at the national and global levels that necessitates the development of potent and safe therapeutics. Existing therapies do not allow curing one third of patients with localized osteosarcoma, the most common type of bone cancer, and three fourths of patients with metastatic disease. The immediate goals of the current proposal are to develop and verify feasibility of a novel strategy, based on the combination of oncolytic viruses and modified proteinaceous toxins, for selective elimination of osteosarcoma cells in vitro and in preliminary animal models. The long-term goal behind the proposal is to convert the strategy into a novel therapeutic platform of improved efficiency and selectivity, tunable against various human cancers. Oncolytic viruses emerged as a powerful tool in anti-cancer therapy. Similarly, the outstanding killing efficiency and selectivity of bacterial toxins has empowered their conversion to immunotoxins. Yet, the broad application of both technologies is restricted by i) a scarcity of truly cancer- specific receptors for delivery of bacterial toxins to cancer cells and ii) heterogeneous response of tumors to clinically relevant oncolytic viruses (HSV1, vaccinia and measles vaccine viruses, etc.). These limitations will be challenged by the central hypothesis of the proposal that the toxicity of oncolytic viruses can be boosted by a novel type of chimeric immunotoxins enabling to increase the range of susceptible to elimination osteosarcoma cancers without compromising the selectivity of targeting. The toxin will be modified in a way to reduce non-specific targeting and assure full control under the therapeutic potency of a selective oncolytic virus. The immediate focus of this proposal will be on the HSV-Q oncolytic derivative of Herpes Simplex Virus and a potent toxin tuned to enter cancer cells via the Anthrax toxin (Atx) entry pathway retargeted to TEM8, EGFR, and HER2 receptors enriched on osteosarcomas and other types of cancer cells. The proposed innovative concept will be thoroughly evaluated in vitro (Aim 1) and its efficiency and selectivity will be tested on cultures of normal cells and osteosarcoma cell lines (Aim2). The data obtained in the Aims 1 and 2 will be applied to conduct preliminary studies on evaluating a combined action of an oncolytic virus and a modified toxin on localized human and mouse osteosarcomas in xeno- and homograft mouse models. If successful, these efforts will lead to a new methodology of exceptional power, selectivity, and safety.